Two-dimensional uv compatible programmable spatial filter

ABSTRACT

Disclosed are mechanisms for selectively filtering spatial portions of light emanating from a sample under inspection within an optical system. In one embodiment, a programmable spatial filter (PSF) is constructed from materials that are compatible with light in a portion of the UV wavelength range. In a specific implementation, the PSF is constructed from a UV compatible material, such as a polymer stabilized liquid crystal material. In a further aspect, the PSF also includes a pair of plates that are formed from a UV grade glass. The PSF may also include a relatively thin first and second ITO layer that results in a sheet resistance between about 100 and about 300 Ω per square. The PSF provides selective filtering in two directions. In other words, the PSF provides two dimensional filtering.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application takes priority under U.S.C. 119(e) of U.S.Provisional Application No.: 60/368,607 filed Mar. 28, 2002 entitled,“UV COMPATIBLE PROGRAMMABLE SPATIAL FILTER” by Dieter E. Wilk, AnlunTang, Eric N. Vella and Rex Runyon which is incorporated by reference inits entirety for all purposes.

BACKGROUND OF THE INVENTION

[0002] The present invention relates generally to optical inspectionsystems for detecting defects on a sample. More specifically, it relatesto mechanisms for filtering noise from the detection of defects withinsuch optical systems.

[0003] Many instruments currently available for detecting smallparticles on wafers, reticles, photo masks, flat panels and otherspecimens use darkfield imaging. Under darkfield imaging, flat, specularareas scatter very little signal back at the detector, resulting in adark image, hence the term dark field. Surface features and objects thatprotrude above the surface scatter more light back to the detector. Indarkfield imaging, the image is normally dark except areas whereparticles or circuit features exist. A darkfield particle detection

[0004] In darkfield type optical inspection systems, an intense lightbeam in the visible wavelength range is directed towards a sample. Lightscattered from the sample in response to such incident beam is thencollected by a detector. The detector generates an image of the samplefrom the scattered light. Since defects, such as particles or voids,cause the incident light to scatter, scattered light may indicate thepresence of such a defect. However, other features of the sample thatare not defects may cause the incident beam to scatter resulting in thedetection of “false” or “nuisance” defects. For example, repeatingpatterns on the sample, which are typically present on a semiconductordevice, cause incident light to scatter so that sharp bright spots areimaged on the detector. These sharp bright spots may obscure actualdefects. Additionally, line features at specific angles on a sample mayresult in scattering in broad regions of the image at specific angles,e.g., 45° and 90°, which also obscures detection of “real defects.”

[0005] There are instruments that address some aspects of the “nuisancedefect” problems associated with darkfield. One method in use today toenhance the detection of particles is spatial filtering. Under planewave illumination, the intensity distribution at the back focal plane ofa lens is proportional to the Fourier transform of the object. Further,for a repeating pattern, the Fourier transform consists of an array oflight dots. By placing a filter in the back focal plane of the lenswhich blocks out the repeating light dots, the repeating circuit patterncan be filtered out and leave only non-repeating signals from particlesand other defects under certain ideal conditions.

[0006] Although conventional Liquid Crystal type spatial filters workwell within inspection systems that operate in the visible light range,they fail to effectively inhibit light in the ultraviolet (UV) regionfrom nuisance sources from reaching the detectors. A UV light source maybe used for any number of reasons, e.g., to effectively detect smallersized defects. However, a conventional spatial filter's extinctioncapability is greatly reduced when going from a visible to an UV lightsource. Additionally, a conventional spatial filter fails to effectivelytransmit scattered light in the UV region. In a specific example, aconventional PSF has an extinction value of 200:1 and a transmissionvalue of 20 percent at the UV wavelength of 364 nm. It should be notedthat the extinction ratios described herein are measured with a detectorthat is at a distance of 10 inches from the filter and has an apertureof 1 centimeter. Finally, both the transmission and extinctionperformance of conventional spatial filters degrade over time under UVlight exposure.

[0007] Accordingly, there is a need for an improved liquid crystal typeprogrammable spatial filter for use in a darkfield optical inspectionsystem that has improved extinction and transmission performance in theUV region.

SUMMARY OF THE INVENTION

[0008] Accordingly, mechanisms are provided for selectively filteringspatial portions of light emanating from a sample under inspectionwithin an optical system. In one embodiment, a programmable spatialfilter (PSF) is constructed from materials that are compatible withlight in a portion of the UV wavelength range. In a specificimplementation, the PSF is constructed from a UV compatible material,such as a polymer stabilized liquid crystal material. In a furtheraspect, the PSF also includes a pair of plates that are formed from a UVgrade glass. The PSF may also include a relatively thin first and secondITO layer that results in a sheet resistance between about 100 and about300Ω per square. The PSF provides selective filtering in two directions.In other words, the PSF provides two dimensional filtering.

[0009] In one embodiment, an optical inspection system for detectinganomalies on a sample is disclosed. The inspection system includes alight source for directing an incident light beam onto a sample and aprogrammable spatial filter (PSF) arranged in a path of light emanatingfrom the sample in response to the incident light beam. The PSF isconfigurable to inhibit or transmit one or more selected portions of theemanating light, and the selected portions of the emanating light may beselected from a plurality of emanating light portions arranged along afirst direction, as well as in a second direction. The first directiondiffers from the second direction. The inspection system furtherincludes a detector arranged within the path of the emanating light sothat the second portion of the emanating light that is transmitted bythe PSF impinges on the detector to thereby form an image of at least aportion of the sample and an analyzer for receiving the image anddetermining whether there are any defects present on the sample portionby analyzing the received image.

[0010] In one implementation, the first direction is perpendicular tothe second direction. In a further aspect, the sub-regions are arrangedto facilitate inhibition of portions of the emanating light at aparticular angle resulting from patterns on the sample. For example, theangle is 45 or 90 degrees from a plane of incidence. In a specificimplementation, the PSF has a pair of plates formed from a material thatsubstantially transmits ultraviolet light and are sized to cover anaperture of the emanating light, the plates being arranged parallel toeach other, a first indium tin oxide (ITO) layer deposed on a first oneof the plates, and a second ITO layer deposed on a second one of theplates. The first and second ITO layer are positioned between theplates. The first and/or the second ITO layer is divided into portionsalong both the first and second directions. The liquid crystal layer isarranged between the first ITO layer and the second ITO layer portions.

[0011] In a further embodiment, the first ITO layer is divided intoportions and the second layer is not. The system further includes acontroller configured for selectively apply a voltage potentialdifference between at least a one of the first ITO layer portions andthe second ITO layer so that an adjacent portion of the liquid crystallayer allows transmission of a first portion of the emanating lightwhile another portions of the liquid crystal layer on which a voltagepotential difference is not applied inhibits a second portion of theemanating light through the PSF. In one aspect, the voltage potentialdifference is greater than a predetermined threshold, and the adjacentportion of the liquid crystal layer allows transmission when a voltagepotential difference is applied that is greater than the predeterminedthreshold. In an alternative embodiment, the voltage potentialdifference is less than a predetermined threshold, and the adjacentportion of the liquid crystal layer allows transmission when a voltagepotential difference is applied that is less than the predeterminedthreshold.

[0012] In a specific embodiment, the first ITO layer is divided into aplurality of portions that are arranged in a plurality of rows and thesecond ITO layer is divided into a plurality of columns. The systemincludes a controller configured to apply a first voltage potentialdifference between a selected row and a selected column so that aportion of the liquid crystal layer that is adjacent to an intersectionof the selected row and column changes from being configured to inhibitto being configured to allow a corresponding portion of the emanatinglight. In a further aspect, the controller is further configured toapply a second voltage potential difference between the selected row andthe selected column that previously had a first voltage applied therebetween so that the portion of the liquid crystal layer that is adjacentto the intersection of the selected row and column remains configured toallow the corresponding portion of the emanating light. The secondpotential difference is lower than the first potential difference. Inyet a further aspect, the controller is further configured to apply athird voltage potential difference between the selected row and theselected column that previously had a second voltage applied therebetween so that the portion of the liquid crystal layer that is adjacentto the intersection of the selected row and column changes from beingconfigured to allow to being configured to inhibit the correspondingportion of the emanating light. The third potential difference is lowerthan the second potential difference. In a further implementation, thecontroller is further configured to apply a fourth voltage potentialdifference between the selected row and the selected column thatpreviously had a third voltage applied there between so that the portionof the liquid crystal layer that is adjacent to the intersection of theselected row and column remains configured to inhibit the correspondingportion of the emanating light. The fourth potential difference ishigher than the third potential difference. In a preferred embodiment,the liquid crystal layer is a bi-stable material.

[0013] In another embodiment, the invention pertains to a method ofdetecting anomalies on a sample. An incident light beam is directed ontoa sample. A first spatial portion of a emanating light beam, whichresults from the incident light beam hitting the sample, is selectivelytransmitted to a detector positioned within an image plane of suchemanating light beam. The first spatial portion is transmitted byapplying a first voltage potential difference between a first row and afirst column of a programmable spatial filter (PSF) having a pluralityof addressable rows and columns. A second spatial portion of theemanating light beam is inhibited through the PSF to the detector.

[0014] In a further aspect, the transmission of the first spatialportion of the emanating light is maintained by applying a secondvoltage potential difference between the first row and the first column.The second voltage potential difference is lower than the first voltagepotential difference. In a further aspect, the first spatial portion ofthe emanating light is inhibited through the PSF from reaching thedetector by applying a third voltage potential difference to the firstrow and the first column. The third voltage potential difference islower than the second voltage potential difference. In another aspect,the inhibition of the first spatial portion of the emanating light ismaintained by applying the second voltage potential difference betweenthe first row and the first column. In yet another embodiment, thesecond spatial portion of the emanating light beam is inhibited fromreaching the PSF the detector is accomplished by applying a secondvoltage potential to a second row and a second column, where the secondvoltage being lower than the first voltage.

[0015] These and other features and advantages of the present inventionwill be presented in more detail in the following specification of theinvention and the accompanying figures which illustrate by way ofexample the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 is a simplified diagrammatic illustration of a darkfieldoptical inspection system in accordance with one embodiment of thepresent invention.

[0017]FIGS. 2A and 2B illustrate a side view and a top view,respectively, of the programmable spatial filter of FIG. 1 in accordancewith one embodiment of the present invention.

[0018]FIG. 3 is a illustration of a segmented ITO layer of aprogrammable spatial filter in accordance with an alternative embodimentof the present invention.

[0019]FIGS. 4A and 4B illustrate a perspective view and a top view,respectively, of a first and second ITO layer of a programmable spatialfilter in accordance with a two dimensional embodiment of the presentinvention.

[0020]FIG. 5 is a graph of transmission as a function of the voltageapplied between any of the intersections of the ITO layer of FIG. 4A and4B in accordance with one embodiment of the present invention.

[0021]FIG. 6 illustrates an example of scattered light from a sample asa function of the x direction.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

[0022] Reference will now be made in detail to a specific embodiment ofthe invention. An example of this embodiment is illustrated in theaccompanying drawings. While the invention will be described inconjunction with this specific embodiment, it will be understood that itis not intended to limit the invention to one embodiment. On thecontrary, it is intended to cover alternatives, modifications, andequivalents as may be included within the spirit and scope of theinvention as defined by the appended claims. In the followingdescription, numerous specific details are set forth in order to providea thorough understanding of the present invention. The present inventionmay be practiced without some or all of these specific details. In otherinstances, well known process operations have not been described indetail in order not to unnecessarily obscure the present invention.

[0023]FIG. 1 is a simplified diagrammatic illustration of a darkfieldoptical inspection system 100 in accordance with one embodiment of thepresent invention. The illustrated inspection system 100 of FIG. 1 onlyincludes features that will facilitate discussion of the embodiments ofthe present invention. FIG. 1 does not include several typicalcomponents of an inspection system so as to not obscure the presentinvention. Additionally, the relative dimensions of the components andplacement of such components of FIG. 1 may be distorted so as to betterillustrate embodiments of the present invention. As shown, theinspection system 100 includes a light source 102 for generating anddirecting a light or electromagnetic beam 104 onto a surface of sample105.

[0024] The sample may be any suitable device or structure that may beinspected for defects using optical inspection techniques. By way ofexamples, the sample may include wafers, reticles, photo masks, flatpanels and other specimens. The defects may include any suitable defectthat is detectable through optical inspection techniques. Defects thattend to scatter light are detectable through darkfield techniquesinclude, but are not limited to, particles on the sample and voidswithin the sample surface.

[0025] A portion of the incident beam is reflected off the sample 105 asreflected beam 106. Additionally, portions of the beam are scattered offthe sample 105 as scattered light 108. Various characteristics of thesample may result in portions of the incident beam 104 being scatteredas scattered light 108. Particular types of defects (e.g., particles orvoids on the sample surface) may cause the incident beam to scatter.Repeating patterns on the sample that are not defects may also cause theincident beam to scatter. For example, typical integrated circuitrepetitive designs may causes the incident light to scatter.

[0026] The scattered light 108 is directed through a first lens 110 thatoutputs collimated scattered light 112. Any suitable mechanism may beutilized to collect the scattered light, besides the first lens 110. Forexample, a pair of side mirrors may be used to direct the scatteredlight 108 towards a programmable spatial filter (PSF) 114. As shown, thefirst lens 110 collects the scattered light 108 and directs it towardsPSF 114 which is positioned within the Fourier transform plane. Althoughthe PSF is described herein as filtering “scattered” light, of course,the PSF may filter any type of light, such as reflected light.

[0027] In general terms, the PSF 114 is configurable to effectivelyinhibit one or more portions of the scattered light and effectivelytransmit portions of the scattered light for a light source within theUV wavelength range. Preferably, the PSF has a minimum extinction valueof about 400:1 and a minimum transmission value of about 40 percent foran incident light beam having a wavelength between about 340 nm andabout 400 nm. Most preferably, the PSF has a minimum extinction value ofabout 500:1 and a minimum transmission value of about 50 percent. Theportions of the scattered light that are inhibited are substantiallyprevented from reaching the second lens 118, while scattered light thatis transmitted substantially reaches the second lens 118.

[0028] Any suitable optical inspection system may be modified toincorporate one or more of the programmable spatial filter embodimentsof the present invention. For example, the spatial filtered inspectionmechanisms of the present invention may be combined with any othersuitable optical inspection system. In a specific implementation, aninspection tool may include darkfield inspection capabilities andbrightfield inspection capabilities. Several embodiments of acombination darkfield and brightfield system are described in U.S. Pat.No. 5,822,055 by Tsai et al., issued Oct. 13, 1998 and U.S. Pat. No.6,201,601 by Vaez-Iravani et al., issued 13 Mar. 2001, which patents arecommonly assigned and herein incorporated by reference in theirentirety. These inspection systems may be easily modified to include aspatial filter of the present invention.

[0029] Referring back to FIG. 1, the second lens is configured to directthe transmitted scattered light 120 towards detector 122 positionedwithin the image plane. The detector is configured to generate an imageof at least a portion of a sample that corresponds to the scatteredlight. The resulting image may then be received into an analyzer 124,where the analyzer 124 determines whether there are any defects presenton the sample portion by analyzing the received image. The analyzer orcontroller 124 may be configured to perform any suitable techniques ofthe present invention, as well as typical control or monitoringmechanisms of the inspection system 100. By way of examples, theanalyzer or controller 124 may be configured to configure variouscomponents of the inspection system, such as the detector 122, the PSF114, and the light source 102, or to monitor characteristics of variouscomponents.

[0030] The PSF may be constructed in any suitable manner so as toachieve effective extinction and transmission performance under UV lightconditions (e.g., the above described extinction and transmissionvalues). FIGS. 2A and 2B illustrate a side view and a top view,respectively, of the programmable spatial filter 114 of FIG. 1 inaccordance with one embodiment of the present invention. In oneimplementation, the PSF is sized to cover the aperture of the scatteredlight. As shown, the PSF includes a pair of plates 202 and 210. Theplates are formed from a material that substantially transmitsultraviolet light, i.e., the plates are each UV compatible. In oneimplementation, the plates are formed from UV grade glass. In theillustrated embodiment, the plates are arranged parallel to each otherand sized to cover an aperture of the scattered light.

[0031] The PSF 114 also includes a first indium tin oxide (ITO) layer204 and a second ITO layer 208 formed from a plurality of ITO layerportions 208 a through 208 i deposed on a first one of the plates 202and a second one of the plates 210, respectively. The ITO layers areconductive and transmit UV light effectively. Of course, any suitablematerial may be used, instead of ITO, that is both conductive andtransparent. The first and second ITO layers 204 and 208 are deposed oneach of the plates 202 and 210, respectively. An ITO layer or patternmay be more easily deposited onto the plate if the plate is formed fromglass, as compared to when the plate is formed from a conventionalplastic. In one embodiment shown in FIG. 2B, the second ITO layerportions 208 form a plurality of parallel strips 208 a through 208 i. Inone embodiment, the ITO layers are relatively thin so as to allowacceptable performance under UV conditions. A thickness that results ina sheet resistance between about 100 and about 300Ω per square hasacceptable conductance and transmission capabilities for UV light.

[0032] The PSF also includes a material 206 between the first and secondITO layers that is selectively configurable to provide UV compatibletransmission and opacity performance. For example, a polymer stabilizedliquid crystal material may be used. Such a liquid crystal material maybe obtained from any suitable company that specializes in liquid crystalapplications, such as AlphaMicron or Kent Optronics, both of Kent, Ohio.Polymer stabilized liquid crystal materials are described farther in thearticle “Assemble-Averaged Dynamic Light Scattering fromPolymer-Stabilized Liquid Crystals.” by P. Y. Lui, L. C. Chien, Mol.Crys. Liq. Cyrs. 348, 187-205 (2000) and U.S. Pat. No. 5,691,795 byDoane et al., issued 25 Nov. 1997, which article and patent areincorporated herein by reference in their entirety.

[0033] Each of the second ITO layer portions 208 are coupled to amechanism for selectively applying a voltage potential differencebetween one or more of the second ITO layer portions 208 and the firstITO layer 204. As shown in FIG. 2B, ITO layer portion 208 a is coupledto switch S10, while ITO layer portion 208 i is coupled to switch S1.Each of the switches S1 through S10 are coupled to voltage source V1.The voltage source V1 is also coupled to the first ITO layer 204. Thus,the voltage V1 may be applied selectively between one or more second ITOlayer portions 208 and the first ITO layer 204. The voltage may beapplied selectively (or selectively not applied) to one or more of thesecond ITO layer portions 208 by any suitable mechanism. In theillustrated embodiment, the controller 124 of FIG. 1 is configured tocontrol the switches S1 through S10 of FIG. 2B.

[0034] When substantially no voltage potential difference is appliedbetween one of the second ITO portions 208 and the first ITO layer 204(or when the voltage difference drops below a predetermined threshold),the portion of the liquid crystal material 206 that is positionedbetween the selected second ITO portion 208 and the first ITO layer 204generally becomes opaque to the scattered light going through suchliquid crystal portion. In an alternative implementation, a voltagedifference above a predetermined threshold may cause the liquid crystalmaterial to become opaque. This opacity causes a corresponding spatialportion of the scattered light to be inhibited from reaching thedetector. The opaque portion of the liquid crystal causes a substantialportion of the scattered light directed through the “deactivated” secondITO portion (e.g., 208 i) to be scattered away from the detector. Asshown in FIG. 2A, S1 is open so that a voltage potential difference isnot applied between second layer portion 208 i and the first layer 204to thereby cause a portion 212 of the liquid crystal 206 to becomeopaque. In contrast, S2 is closed so that a voltage potential differenceis applied between the second layer portion 208 h and the first layer204 to thereby cause a portion 214 to become clear. Any number of thesecond ITO layer portions 208 may be selectively switched to not bedecoupled from voltage V1 so that corresponding portions of liquidcrystal 206 may become opaque and inhibit different spatial portions ofthe scattered light from the sample.

[0035] In one embodiment, the ITO layer portions 208 are selectivelydeactivated to inhibit certain spatial scattered light portions thatresult from repeating patterns on the sample, rather than defects. Thatis, the ITO layers are selected to correspond to portions of thescattered light that contain “nuisance” defects. These inhibitedportions of the scattered light may include sharp, bright diffractionspots resulting from repeating patterns on the sample Alternatively, theinhibited spatial portions may include scattered light at particularangles with respect to the plane of incidence, such as 45 and 90degrees, that result from repeating lines on the sample. A selectionexample is described further below with reference to FIG. 6.

[0036]FIG. 3 is a illustration of a first segmented ITO layer 304 and asecond ITO layer 302 of a PSF in accordance with an alternativeembodiment of the present invention. These ITO layers may be used toform part of a programmable spatial filter, such as the PSF illustratedin FIG. 2A. That is, the ITO layers 304 and 302 of FIG. 3 may replacethe ITO layers 204 and 208 of FIG. 2A. All the additional featuresdescribed above in relation to FIGS. 2A and 2B may be used within a PSFwhich includes the ITO layers of FIG. 3. Referring back to FIG. 3, thefirst ITO layer 304 is formed from a plurality of strips 306 a through3061. At least a portion of the strips 306 are “segmented.” As shown,strip 306 d includes segments 308 a through 308 c. Likewise, strips 306e through 306 j are each also segmented. Each strip or segment may beindividually deactivated. For instance, a voltage difference may bedecoupled from between segment 308 a and the second ITO layer 302.Likewise, a voltage difference may be decoupled from between segment 308b and the second ITO layer 302. A voltage difference may also bedecoupled from between any of the strips that are not segmented. Forinstance, a voltage difference may be decoupled from between strip 306 aand the second ITO layer 302.

[0037] Segmentation of one of the ITO layers allows greater flexibilityin inhibiting different spatial portions of the scattered light. Inother words, the PSF can be configured to inhibit selected spatialportions of the scattered light in two directions (e.g., x and y). ThePSF embodiments of FIGS. 2A and 2B merely selectively inhibit spatialportions of the scattered light in one direction. That is, one mayselectively inhibit one or more strips of scattered light in a singledimension or direction. In contrast, the embodiment of FIG. 3 allows oneto selectively inhibit different portions of scattered light in an xdirection by selecting one or more of the strips (i.e., 306 a through3061) and in a y direction by selecting one or more of the segments(e.g., 308 a, 308 b, and/or 308 c). The segments of the ITO layer may bearranged in any two suitable directions, which do not have to beperpendicular to each other as shown in the illustrated embodiment.

[0038]FIGS. 4A and 4B illustrate a perspective view and a top view,respectively, of a first and second ITO layer of a programmable spatialfilter in accordance with a two dimensional embodiment of the presentinvention. In this embodiment, a two dimensional array of ITO portionsmay be selectively deactivated so that a corresponding two dimensionalarray of liquid crystal portions are selectively turned opaque. The ITOlayers of FIGS. 4A and 4B may be incorporated into the PSF of FIGS. 2Aand 2B. That is, the ITO layers of FIGS. 4A and 4B may be substitutedfor the ITO layers of FIGS. 2A and 2B. All the additional featuresdescribed above in relation to FIGS. 2A and 2B may be used within a PSFwhich includes the ITO layers of FIGS. 4A and 4B.

[0039] As shown, a first ITO layer 404 is formed into a plurality ofstrips or “columns.” A second ITO layer 402 is formed into a pluralityof strips or “rows.” The first ITO layer 404 is positioned over thesecond ITO layer 402. A liquid crystal material 408 is positionedbetween the first ITO layer 404 and the second ITO layer 402. The firstand second ITO layers form a plurality of intersections 406. Eachintersection 406 corresponds to a portion of the liquid crystal material408 that is positioned between the intersection area 406. Of course, thetwo ITO layers may be arranged in any suitable number and type of shapesso as to form a plurality of intersections between pairs of adjacentfirst and second ITO layer portions.

[0040] Referring to FIG. 4B, each intersection may be deactivated bydecoupling or reducing a voltage from a selected row of ITO layer 402and a selected column of ITO layer 404. For example, when a particularvoltage difference that is less than or equal to a predeterminedthreshold is applied between the lower layer row 402 g and the uppercolumn 404 a, intersection 406a is deactivated. That is, the liquidcrystal material that is below the intersection 406 a or between theoverlapping column 404 a and row 402 g becomes opaque. By way of anotherexample, when a voltage potential difference below the predeterminedlevel is applied between row 402 a and column 404 g, the intersection406 c becomes deactivated so that the liquid crystal material below thisintersection 406 c becomes opaque. In an alternative embodiment, anintersection becomes opaque when a voltage difference applied betweenthe overlapping column and row rises above a predetermined threshold.

[0041] Preferably, the liquid crystal material is formed from abi-stable material. FIG. 5 is a graph of transmission as a function ofthe voltage applied between any of the intersections of the ITO layer ofFIG. 4A and 4B in accordance with one embodiment of the presentinvention. At zero voltage potential difference, there is zerotransmission through the ITO intersection. When the voltage differenceis greater than V2, the transmission percentage through suchintersection increases to a plateau, for example, of 60%. Theintersection stays at 60% transmission until the voltage differencefalls below V1. That is, the voltage difference may fall below thevoltage V2 that activated such intersection and still remain “on”.

[0042] Thus, the voltage to turn on a particular intersection may be metby a contribution of voltages applied to both a particular row and aparticular column. In one implementation, the voltage of either thecorresponding row or column may be removed without causing theparticular intersection to switch to the off state. In other words, theintersection is maintained in an on state for a range of voltages (i.e.,V1 through V2) below the initial “turn-on” voltage V2. For example, 50volts may be applied to a first row and 50 volts may be applied to afirst column to activate a particular intersection. The first rowvoltage may be then decreased to zero, while the first column remains at50 volts. If V1 corresponds to 50 volts and V2 corresponds to a 100volts, the intersection will remain on even though it is now at 50volts, instead of 100 volts. This bimodal scheme allows anotherintersection in the same row or column as a particular intersection todeactivate without turning off the particular intersection.

[0043] When the intersection voltage falls below V1, the intersectionswitches to the off state and the corresponding liquid crystal portionbecomes opaque and inhibits a previously transmitted portion of thescattered light to now be blocked from passing through the PSF. Forexample, the above described particular intersection may be turned offby having both voltages of the first row and first column fall below V1or 50 volts (e.g., the first row and column fall to 0 V). A particularintersection will remain off when the voltage at such intersection isincreased to voltages between V1 and V2. Even if the voltage of aparticular row or column goes above V1 or 50 volts, the particularintersection remains off. This bimodal scheme allows anotherintersection in the same row or column as a particular intersection toactivate without turning on the particular deactivated intersection.

[0044] The PSF embodiments of the present invention may be used toselectively mask various portions of the scattered light that correspondto “nuisance defects”, as well as portions of the scattered light thatare known to not contain the “real defect.” FIG. 6 illustrates anexample of scattered light intensity from a sample as a function of thex direction. As shown, portion 601 of the scattered light corresponds tothe defect itself. The scattered light portions 602 a and 602 bcorrespond to “nuisance defects” in the form of bright, sharpdiffraction spots resulting from a repeating pattern on the sample. Thescattered light portion 604 corresponds to a portion of the scatteredlight that is known to not contain a defect. The PSF of the presentinvention may be used to inhibit scattered light from portions 602 a,602 b, and 604 so as to isolate and enhance the actual defect 601.

[0045] Although the foregoing invention has been described in somedetail for purposes of clarity of understanding, it will be apparentthat certain changes and modifications may be practiced within the scopeof the appended claims. Therefore, the described embodiments should betaken as illustrative and not restrictive, and the invention should notbe limited to the details given herein but should be defined by thefollowing claims and their full scope of equivalents.

What is claimed is:
 1. An optical inspection system for detectinganomalies on a sample, comprising: a light source for directing anincident light beam onto a sample; a programmable spatial filter (PSF)arranged in a path of light emanating from the sample in response to theincident light beam, wherein the PSF is configurable to inhibit ortransmit one or more selected portions of the emanating light, whereinthe selected portions of the emanating light may be selected from aplurality of emanating light portions arranged along a first direction,as well as in a second direction, the first direction differing from thesecond direction; a detector arranged within the path of the emanatinglight so that the second portion of the emanating light that istransmitted by the PSF impinges on the detector to thereby form an imageof at least a portion of the sample; and an analyzer for receiving theimage and determining whether there are any defects present on thesample portion by analyzing the received image.
 2. A system as recitedin claim 1, wherein the first direction is perpendicular to the seconddirection.
 3. A system as recited in claim 1, wherein the sub-regionsare arranged to facilitate inhibition of portions of the emanating lightat a particular angle resulting from patterns on the sample.
 4. A systemas recited in claim 1, wherein the angle is 45 degrees from a plane ofincidence.
 5. A system as recited in claim 1, wherein the angle is 90degrees from a plane of incidence.
 6. A system as recited in claim 1,wherein the PSF comprises: a pair of plates formed from a material thatsubstantially transmits ultraviolet light and are sized to cover anaperture of the emanating light, the plates being arranged parallel toeach other; a first indiuim tin oxide (ITO) layer deposed on a first oneof the plates; a second ITO layer deposed on a second one of the plates,wherein the first and second ITO layer are positioned between theplates, wherein the first and/or the second ITO layer is divided intoportions along both the first and second directions; and a liquidcrystal layer arranged between the first ITO layer and the second ITOlayer portions.
 7. A system as recited in claim 6, wherein the first ITOlayer is divided into portions and the second layer is not, the systemfurther comprising a controller configured for selectively applying avoltage potential difference between at least a one of the first ITOlayer portions and the second ITO layer so that an adjacent portion ofthe liquid crystal layer allows transmission of a first portion of theemanating light while another portions of the liquid crystal layer onwhich a voltage potential difference is not applied inhibits a secondportion of the emanating light through the PSF.
 8. A system as recitedin claim 7, wherein the voltage potential difference is greater than apredetermined threshold, and wherein the adjacent portion of the liquidcrystal layer allow transmission when a voltage potential difference isapplied that is greater than the predetermined threshold.
 9. A system asrecited in claim 7, wherein the voltage potential difference is lessthan a predetermined threshold, and wherein the adjacent portion of theliquid crystal layer allow transmission when a voltage potentialdifference is applied that is less than the predetermined threshold. 10.A system as recited in claim 6, wherein the first ITO layer is dividedinto a plurality of portions that are arranged in a plurality of rowsand the second ITO layer is divided into a plurality of columns, thesystem comprising a controller configured to apply a first voltagepotential difference between a selected row and a selected column sothat a portion of the liquid crystal layer that is adjacent to anintersection of the selected row and column changes from beingconfigured to inhibit to being configured to allow a correspondingportion of the emanating light.
 11. A system as recited in claim 10,wherein the controller is further configured to apply a second voltagepotential difference between the selected row and the selected columnthat previously had a first voltage applied there between so that theportion of the liquid crystal layer that is adjacent to the intersectionof the selected row and column remains configured to allow thecorresponding portion of the emanating light, wherein the secondpotential difference is lower than the first potential difference.
 12. Asystem as recited in claim 11, wherein the controller is furtherconfigured to apply a third voltage potential difference between theselected row and the selected column that previously had a secondvoltage applied there between so that the portion of the liquid crystallayer that is adjacent to the intersection of the selected row andcolumn changes from being configured to allow to being configured toinhibit the corresponding portion of the emanating light wherein thethird potential difference is lower than the second potentialdifference.
 13. A system as recited in claim 12, wherein the controlleris further configured to apply a fourth voltage potential differencebetween the selected row and the selected column that previously had athird voltage applied there between so that the portion of the liquidcrystal layer that is adjacent to the intersection of the selected rowand column remains configured to inhibit the corresponding portion ofthe emanating light wherein the fourth potential difference is higherthan the third potential difference.
 14. A system as recited in claim13, wherein the liquid crystal layer is a bi-stable material.
 15. Anoptical inspection system for detecting anomalies on a sample,comprising: a light source for directing an incident light beam onto asample; a programmable spatial filter (PSF) arranged in a path of lightemanating from the sample in response to the incident light beam,wherein the PSF comprises: a pair of plates formed from a material thatsubstantially transmits ultraviolet light and are sized to cover anaperture of the emanating light, the plates being arranged parallel toeach other; a first indium tin oxide (ITO) layer deposed on a first oneof the plates, the first ITO layer being divided into a plurality offirst portions along a first direction; a second ITO layer deposed on asecond one of the plates, wherein the first and second ITO layer arepositioned between the plates, the first ITO layer being divided into aplurality of first portions along a second direction that differs fromthe first direction; a liquid crystal layer arranged between the firstITO layer and second ITO layer portions, the liquid crystal layer beingformed from a bi-stable material; and a controller configured forselectively applying a voltage potential difference between at least apair of first and second ITO layer portions so that an adjacent portionof the liquid crystal layer are configured to inhibit or transmit one ormore first portions of the emanating light; a detector arranged withinthe path of the emanating light so that the portions of the emanatinglight that are transmitted by the PSF impinge on the detector to therebyform an image of at least a portion of the sample; and an analyzer forreceiving the image and determining whether there are any defectspresent on the sample portion by analyzing the received image.
 16. Asystem as recited in claim 15, wherein the liquid crystal is a UVcompatible material.
 17. A system as recited in claim 16, wherein theliquid crystal is a polymer stabilized liquid crystal material.
 18. Asystem as recited in claim 17, wherein the pair of plates are formedfrom a UV grade glass.
 19. A system as recited in claim 18, wherein thefirst and second ITO layers have a thickness value so as to result inbetween about 100 and about 300Ω per square.
 20. A system as recited inclaim 15, wherein the voltage potential difference is greater than apredetermined threshold, and wherein the adjacent portion of the liquidcrystal layer allow transmission when a voltage potential difference isapplied that is greater than the predetermined threshold.
 21. A systemas recited in claim 15, wherein the voltage potential difference is lessthan a predetermined threshold, and wherein the adjacent portion of theliquid crystal layer allow transmission when a voltage potentialdifference is applied that is less than the predetermined threshold. 22.A method of detecting anomalies on a sample, the method comprising:directing an incident light beam onto a sample selectively transmittinga first spatial portion of a emanating light beam, which results fromthe incident light beam hitting the sample, to a detector positionedwithin an image plane of such emanating light beam, wherein the firstspatial portion is transmitted by applying a first voltage potentialdifference between a first row and a first column of a programmablespatial filter (PSF) having a plurality of addressable rows and columns;and inhibiting a second spatial portion of the emanating light beamthrough the PSF to the detector.
 23. A method as recited in claim 22,further comprising maintaining the transmission of the first spatialportion of the emanating light by applying a second voltage potentialdifference between the first row and the first column, the secondvoltage potential difference being lower than the first voltagepotential difference.
 24. A method as recited in claim 23, furthercomprising inhibiting the first spatial portion of the emanating lightthrough the PSF from reaching the detector by applying a third voltagepotential difference to the first row and the first column, wherein thethird voltage potential difference is lower than the second voltagepotential difference.
 25. A method as recited in claim 24, farthercomprising maintaining the inhibition of the first spatial portion ofthe emanating light by applying the second voltage potential differencebetween the first row and the first column.
 26. A method as recited inclaim 22, wherein inhibiting the second spatial portion of the emanatinglight beam from reaching the PSF the detector is accomplished byapplying a second voltage potential to a second row and a second column,the second voltage being lower than the first voltage.
 27. A method asrecited in claim 26, wherein the second voltage is less than the firstvoltage by a predetermined amount when the second spatial portion of theemanating light was previously being inhibited from passing through thePSF.
 28. A method as recited in claim 26, wherein the second voltage isless than the first voltage by a second predetermined amount that isgreater than the first predetermined amount when the second spatialportion of the emanating light was previously being transmitted throughthe PSF.